I believe you should use the accusative for the expression 'each week' каждую неделю. However 'each car' is каждая машина in the nominative. Is this correct and, if so, why?
No, it's not quite right. Depending on context
каждую машину is correct and
каждая неделя is correct as well.
The thing is
неделю in accusative stands for a period of time, so it's just should be memorized that in phrase like:
Всю неделю я пил.
всю неделю stands for
whole week. Correspondingly
каждую неделю stands for "every week" as a period during which something happens.
However, in phrase like:
Тут всегда происходит что-то новое, и каждая неделя особенная.
каждая неделя is also translated as
every week but the difference is that here we are not talking about something that happens every week but about the week as a concept itself - "every week is special".
You confusion is quite understandable, because not all common periodization falls under the same pattern - it's all about nouns in feminine - well, strictly speaking it's just that for non-feminine accusative and nominative are coinciding. Here's more or less full list:
- Каждую секунду, каждую минуту (yep, same pattern)
- Каждую ночь (actually yep, it's just that
ночьin accusative does not change)
- Каждый час, каждый день, каждый месяц, каждый год, каждый век (nope, well, again, in that sense that actually nominative and accusative are equivalent)
By the way, another important form worth to mention is
неделями - it translated "for weeks", so "он квасил неделями" is "he's been binging for weeks". Unlike the single accusative, this plural instrumental pattern works for a bigger set of nouns, so
веками are also all valid.
The Nominative case serves for denoting either the subject or the predicate of a sentence.
Знание - сила, Время - деньги - both the subjects and the predicates are the nouns in the N.c.
In all other functions the objective cases (all the rest) are used.
As "каждую неделю", "год назад", "через пять лет" are the adverbial modifiers of time, they cannot be used in the Nominative case. The listed above are in the Accusative case.
But if we make a phrase with "каждый + time period" a subject, it immediately goes into the Nominative case.
Каждая неделя, проведенная за работой, приближала его к успеху.
Каждый день жизни - радость.